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Home , Balfour Formation, Beaufort Group, Ecca Group, Fort Beaufort, Whitehill Formation

Open Geosci. 2019; 11:313–326

Research Article

Christopher Baiyegunhi*, Zusakhe Nxantsiya, Kinshasa Pharoe, Temitope L. Baiyegunhi, and Seyi Mepaiyeda Petrographical and geophysical investigation of the between Fort Beaufort and Grahamstown, in the Province, South https://doi.org/10.1515/geo-2019-0025 depth slices result, dolerite intrusions are pervasive in the Received Mar 20, 2018; accepted Mar 13, 2019 northern part of the study area, extending to a depth of about 6000 m below the ground surface. The appearance Abstract: The of the Ecca Group in the Eastern of dolerite intrusions at the targeted depth (3000 - 5000 m) was investigated in order to reveal petro- for gas exploration could pose a serious threat to fracking graphic and geophysical characteristics of the formations of the for gas. that make up the group which are vital information when considering fracking of the Karoo for shale gas. The pet- Keywords: Density, dolerites, Ecca Group, gravity, mag- rographic study reveals that the rocks of the Ecca Group netic, porosity are both argillaceous and arenaceous rock with quartz, , micas and lithics as the framework minerals. The are graywackes, immature and poorly sorted, thus giving an indication that the source area is near. The 1 Introduction observed heavy minerals as well as the lithic grains signify This study focuses on road cut exposures of the Ecca Group that the minerals are of granitic, volcanic and metamor- along road between Fort Beaufort and Grahamstown phic origin. The porosity result shows that of all the forma- (Figure 1). The Ecca Group is the second largest subdivi- tions that make up the Ecca Group, the Whitehill Forma- sion of the in ; it covers tion is the most porous with an average porosity of about more than 50% of the ’s land surface and it is 2.1% and also least dense with an average dry density of believed to have been emplaced during the Early . about 2.5 g/cm3. The least porous unit is the Ripon For- The Karoo is a semi-desert region of Southern Africa with mation with porosity of about 0.8% but has the highest vast distribution of sedimentary rocks that covers an area dry density of approximately 2.8 g/cm3. The magnetic map of up to 700 000 km2 [1]. The Karoo Supergroup owes its shows some ring-like structures which coincide with do- name to the Karoo region in which it was accumulated in lerites that were mapped in the field. As revealed by the an environment believed to be a retro-arc environment [2]. To date, there is controversy surround- ing the origin and development of the Karoo Basin. The *Corresponding Author: Christopher Baiyegunhi: Department basin is well known worldwide because of its con- of Geology and Mining, University of Limpopo, Private Bag X1106, tent that record an unbroken stratigraphic succession that Sovenga 0727,South Africa; Department of Geology, Faculty of span from the to the Middle . Re- Science and Agriculture, , Private Bag X1314, cently, it was envisaged that the Karoo basin developed Alice, 5700, Eastern Cape Province, South Africa; Email: [email protected] due to blocks subsidence along the marginal faults [3, 4]. Zusakhe Nxantsiya, Kinshasa Pharoe: Department of Geology, The Ecca Group is a sedimentary geological unit that Faculty of Science and Agriculture, University of Fort Hare, Private comprises mostly of and sandstones. Almost all of Bag X1314, Alice, 5700, Eastern Cape Province, South Africa; Council South Africa’s resources and one-third of the coal re- for Geoscience, 280 Pretoria Street, Silverton, Pretoria 0184, South sources in the southern hemisphere are in the rocks of the Africa Ecca Group [5]. The coal seams is described to be horizon- Temitope L. Baiyegunhi, Seyi Mepaiyeda: Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private tal throughout in the Main Karoo Basin except where the Bag X1314, Alice, 5700, Eastern Cape Province, South Africa seams are associated with dolerite intrusions [1]. The dis-

Open Access. © 2019 C. Baiyegunhi et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License 314 Ë C. Baiyegunhi et al.

Figure 1: Geological map of the study area (Modified from [1]). The inserted small red colour box shows the study area.

Table 1: of the Ecca Group in the Eastern Cape Province of South Africa [1].

Group Formation Lithology Special Characteristics Ages Ecca Waterford , Water escapes structures, slumping, ball and 258 - 252 Ma pillow structures Fort Brown Sandstone shale, Rhythmites 262 - 258 Ma Ripon Sandstone, shale Wave ripples, , rhythmites 267 - 262 Ma Collingham Shale, claystones, , Thinly bedded, yellow tuff, Glossopteris, trace 274 - 270 Ma , sandstones Whitehill Black carbonaceous Weathered white shale , pyritic, at 275 Ma shale, surface, dolomite Prince Albert Khaki shale, siltstones , quartz veins, pyritic, sharks teeth, 280 - 275 Ma foraminifera Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 315 tribution, lateral extent, thickness and maceral content roo Supergroup [13]. The term Ecca was suggested by Ru- of these coal seams are controlled by basin tectonic and bidge (1858) in [13] for argillaceous sedimentary strata ex- differential subsidence [1, 6]. The Ecca group is also be- posed in the Ecca Pass, near Grahamstown in the Eastern lieved to be hosting the much debated shale gas with more Cape Province, South Africa. Thus, the use of the term Ecca emphasises on the Whitehill and Collingham Formations outside the Main Karoo Basin is sometimes questionable in the southern Karoo. Since the discovery of natural gas as the rock types could be totally different. The group com- deposit in the carbonaceous shale of the Whitehill and prises of shale, , siltstones, sandstones, minor Collingham Formations, the Ecca Group has become fa- and coal. mous; however, local exploration started in the 1960s but Generally, the Karoo Supergroup is stratigraphically was later stopped in the late 1970s due to poor technical subdivided into five main groups, namely; the Dwyka know-how [7, 8]. (Late Carboniferous), Ecca (Early Permian), Beaufort (Late Shale gas has become important in the last few Permian–Middle ), Stormberg (Late Triassic–Early and the main exploration target is the Ecca Group due to Jurassic) and Groups (Middle Jurassic) [5]. the recent developments in technology on how to recover The Drakensberg lavas are believed to have terminated sed- the natural gas from the host shale. Fracking or fractur- imentation in the basin in the Middle Jurassic [14]. The ing of the Karoo to extract shale gas has attracted more rocks of the Ecca Group were first identified by [15] as interest from researchers and economists. This is due to the “Ecca Beds” and were separated from the overlying the credence that shale gas is an economically feasible fos- “Beaufort Beds” (now known as the ). The sil fuel for generating cheap and clean electricity. Thus re- “Ecca Beds” were at a later stage renamed the “Ecca Se- ducing environmental pollution as well as help in meeting ries” by [16] in his fourfold sub-division of the “Karoo Sys- the increasing demands of electricity in South Africa. This tem”. [17] subdivided the Ecca “Series” in the southern study was undertaken in order to provide more informa- Karoo Basin into a Lower, Middle and Upper Ecca Stage. tion on the petrographic and geophysical characteristics The Ecca Series in the southern Karoo Basin was later re- of the Ecca Group in the Eastern Cape Province of South named by [18] as the Ecca Group. At the same time, the Africa. Lower, Middle and Upper Ecca "Stages" became formal- ized as the Ripon, Fort Brown and Waterford Formations. The underlying Prince Albert and Whitehill Formations 1.1 Geological background were also included into the Ecca Group as the formations that make up the Lower Ecca Group, although they had The Main Karoo Basin of South Africa hosts the thickest been part of the upper Dwyka. [18] later renamed the grey and stratigraphically most complete mega- sequence of and yellow shales on top of the as the several depositories of the Permo-Carboniferous to Juras- Collingham Formation. With slight alterations this strati- sic age in southwestern continent [2]. graphic outline was received by the South African Com- Several models have been proposed for the formation of mittee for [19] and to date this stratigraphic the basin. The interpretation of the evolution and tec- outline for the southern Karoo Basin is used. The stratigra- tonic setting of the Karoo Basin vary from a transient phy of the Ecca Group is however complex and this com- hypothetical mantle plume related model [9], a retro-arc plexity is related to its mode of origin, deep marine wa- foreland basin formed as a result of shallow angle sub- ter and shallow marine turbidites and submarine fan de- duction of the palaeo-Pacific plate underneath the Gond- posits from specific source areas [13]. The marine clays wana supercontinent [1], a transtensional foreland system and mudstones of the Prince Albert Formation were de- formed as a result of subsidence and tilting in a strike- posited on the diamictites of the in the south- slip regime [3, 4], extensional back-arc basin formed due eastern part of the Karoo Basin [20]. This was followed by to oblique of the palaeo-Pacific plate under the carbonaceous shale of the Whitehill Formation. Sub- western Gondwana [10], to a thin-skinned fold belt that sequently, the Collingham Formation that is made up of developed from collisional and distant subduc- persistent grey shales alternating with yellow-claystones, tion to the south [11]. The maximum preserved thickness as well as the sandstones and shales of the Ripon, Fort of these mega-sequence that is adjacent to the Cape Fold Brown and Waterford Formations were deposited on the Belt is in excess of 6 km [12]. The Ecca Group is a rock se- submarine fans, shelf and deltas, respectively [18]. The quence that accumulated between the Late Carboniferous Dwyka and Ecca Groups were deposited during the seaway Dwyka Group and the Late Permian-Middle Triassic Beau- transgression into the interior part of the southern Karoo fort Group, occupying most of the Permian time of the Ka- Basin [21]. Nevertheless, at the end of Ecca time, complete 316 Ë C. Baiyegunhi et al. regression in the Ecca Group occurred from the limits of ham Formation by [26], namely mudstone, and the preserved basin [2]. The sedimentary fill of the basin very fine grained sandstone, chert, terrigenous rock, phos- accumulated under the influence of tectonics and climate phorite and K-bentonite (altered ash fall tuff). The oc- changes [2, 22]. Thus, it reflects variable environments currence of thin siltstone and sandstone layers is a re- from glacial to deep marine, fluvial, deltaic, and aeolian sult of low density, distal turbidity currents. Small con- regimes [23]. The deeper marine of the Karoo Super- volutions and partly striations are found in the forma- group (i.e. Dwyka and early Ecca Groups) accumulated dur- tion [1]. The trace fossils that have been found in asso- ing the under-filled phase of the foreland system, while the ciation with the mudstone facies are mostly worm bur- shallow marine facies of the Karoo Supergroup (late Ecca rows Scolia and , which are trails formed by gas- Group) is related to the filled phase of the basin [2, 22], tropods [26]. The Ripon Formation consists of sandstones, which was followed by an overfilled phase characterised mudstone and carbonaceous shale. It is poorly sorted, by more fluvial [24]. The changes in fluvial with fine- to very fine-grained lithofeldspathic sandstone style in the Karoo Basin at the end of the Permian were ac- alternating with dark grey clastic rhythmite and mud- tivated by a pulse of thrusting in the southerly source area stone [1]. The Fort Brown Formation lies conformably on dated at about 247 Ma [2, 22]. the Ripon Formation and ranges in thickness from 500 – 1500 m representing a thick, relatively homogenous argillaceous sequence that presently has no stratigraphic 1.2 Lithostratigraphy of the Ecca Group subdivisions [19]. The Fort Brown Formation comprises of dark-grey shale and siltstone, with well-developed varved The Ecca Group in the Eastern Cape Province of South rhythmite. Stratigraphically, the lower Fort Brown Forma- Africa can be stratigraphically subdivided into five forma- tion is more argillaceous, and has been proposed to rep- tions, namely; the Prince Albert, Whitehill, Collingham, resent in a pro-delta setting [1]. The Ripon and Ripon, and Fort Brown Formations [18, 19, 24]. The Prince Fort Brown Formations represent a turbidite fan complex Albert Formation consists mostly of grey mudstone, in the with the direction of movement from the southeast in a form of olive-grey laminated shales, less laminated and deep to medium aqueous [27]. more siliceous olive grey shale and some sandstone beds The Waterford Formation conformably overlies the near the base [1]. Carbonaceous shales which are found Fort Brown Formation and it varies from about 150 – 580 around 5m from the top of the formation are dark green m in thickness, depending on the locality, the formation is to black in colour [23]. Sandy beds have been noticed near also characteristically more arenaceous than the underly- the top of the formation as well as a soft brownish ma- ing Fort Brown Formation [19]. The lithofacies of the Water- terial having an abundance of iron oxide [23]. The fine ford Formation are mostly arenaceous and their internal grained mudstones were deposited by suspension settling geometries and structure, including current and wave in- whereas sandstone beds were deposited as a result of tur- duced cross-bedding, scour and fill features, planar beds, bidity currents [1]. The Whitehill Formation is also known and ripple marks, coupled with the lack of desiccation fea- as the “white band” due to its white appearance on the sur- tures, all point to sub-aqueous deposition [19]. The fact face which resulted from the of black that the facies associations are repeated several times in bearing shales into white soft sediments. The formation vertical succession and appear to exhibit a degree of cyclic- is up to 60 m thick, with lenses of dolomite and organic ity is suggestive of delta lobe switching [19]. The rocks form- rich layers [23]. The carbonaceous shale is finely laminated ing the base of the Waterford Formation are therefore con- and probably formed in a highly anoxic environment as sidered to have been deposited in a delta front environ- suggested by [25] and there is a likelihood that the sedi- ment as previously proposed by [5, 20, 25]. The Ecca Group ments of the Whitehill Formation were deposited on an rests directly on the pre-Karoo surface in some places, in- unstable slope, where slumping would have taken place. dicating a significant post-Dwyka Group topography and This would have resulted in an environment where there localized pre-Ecca , such as non-deposition or ero- was large influx of carbonaceous matter. Contemporane- sion of the Dwyka Group [28]. ous faulting and folding of the Whitehill Formation is de- scribed as a “decollement”, and these are significant fea- tures of the Whitehill Formation [1]. The Collingham Formation that overlies the Whitehill Formation comprises of shale, mudstone, and siltstone. About six lithofacies have been documented in the Colling- Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 317

Figure 2: Mass of rock sampled being measured in the laboratory using Adam electronic weighing balance.

Figure 3: Thin section photomicrograph of the shales from the lower Ecca Group (a) Detrital grains of the Prince Albert Formation; (b) Mineral grains lying parallel to the lamination planes or cleavage in shale from the Whitehill Formation; (c) Detrital quartz grains of the Whitehill Formation; (d) Lamination /alignment of grains of the Collingham Formation. 318 Ë C. Baiyegunhi et al.

Figure 4: Thin section photomicrograph of the sandstones from the upper Ecca Group (a) Feldspathic graywackes of the Ripon Formation, red arrow depict polycrystalline quartz with two quartz grains; (b) Feldspar lithic and albite in sandstone from the Ripon Formation; (c) Framework minerals of the Fort Brown Formation, red arrow depict feldspar grains undergoing alteration to kaolinite; (d) Quartz and volcanic lithics of the Fort Brown Formation.

Table 2: Average dry, wet and grain densities of the rock samples.

FORMATION LITHOLOGY NUMBER AVERAGE AVERAGE AVERAGE AVERAGE OF DRY DENSITY WET DENSITY PARTICLE DENSITY POROSITY SAMPLES (g/cm3) (g/cm3) (g/cm3) (%) Fort Brown Shale 8 2.7564 2.7663 2.7835 1.1238 Ripon Shale 10 2.7615 2.7707 2.7867 0.8773 Collingham Shale / tuff 10 2.6978 2.7122 2.7363 1.1175 Whitehill Black shale 14 2.5258 2.5596 2.6102 2.1023 Prince Albert Khaki shale 10 2.6411 2.6540 2.6748 1.2601

2 Materials and methods and recorded as dMb. Then for the saturated density, sam- ples were soaked in water for 24 hours so that the pores spaces are saturated or completely filled with water, then A total of 52 rock samples were collected along road cut ex- the soaked sample was submerged in water and the read- posure for density measurements and petrographic study. ing is quickly taken and recorded as dMc. More details The Archimedes method was used to determine the densi- on the procedures for density measurement can be found ties of the rock samples. The porosity was calculated from in [29, 30]. the measured density values. The rock samples were sun The mathematically expression to calculate dry, wet dried for 24 hours in order to remove any moisture from and grain density and porosity are shown below: the voids. The dry sample was placed on the weighing bal- [︂ ]︂ ance and the mass in air was recorded as dMa (Figure 2). (︀ )︀ dMa Dry density ρd = × ρw (i) The same sample was placed on the loop that was im- dMa − dMb mersed in the water and the reading was quickly taken Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 319

Figure 5: Bar chart showing densities and porosity of 10 shale samples from the Prince Albert Formation. Figure 8: Bar chart showing densities and porosity of 10 sandstone samples from the Ripon Formation.

Figure 6: Chart of densities and porosity of 14 carbonaceous shale samples from the Whitehill Formation. Figure 9: Chart showing densities and porosity of 8 sandstone samples from the Fort Brown Formation.

Figure 7: Chart of densities and porosity of 10 shale samples from the Collingham Formation. Figure 10: Average densities - porosity relationship for rocks from the Ecca Group. The negative correlation indicates that the lower the densities, the higher the porosity. [︂ ]︂ (︁ )︁ dMa Grain density ρg = × ρw (ii) dMa − dMc where dMa = mass of dry sample in air; dMb = mass of sam- [︂ ρ ]︂ ple in water; dMc = mass of wet sample in water and ρw = Porosity(Φ) = 1 − d × 100% (iii) density of water; ρd = dry density and ρp = grain density; ρp ρwet = wet density; Φ = porosity. [︀ ]︀ Wet density (ρwet) = ρd + Φ × ρw (iv) 320 Ë C. Baiyegunhi et al.

nature. The matrix minerals are muscovite, sericite, clay minerals (kaolinite, smectite, illite), feldspar and quartz. Most of the matrix minerals are formed due to recrystal- lization. Hematite and pyrite are the observed heavy miner- als. The arenaceous (sandstones) of the upper Ecca Group are mostly graywackes (Figure 4). The sandstones are poorly sorted and dominated by fine grained matrix. The framework grains are quartz, feldspar and lithics. Based on the proportion or ratio of the feldspar to lithic frag- ments, the sandstones of the upper Ecca Group are termed arkosic sandstones. Feldspathic graywackes are dominant Figure 11: Variation of average densities and porosity with formation in the Ripon Formation and lithic graywackes (quartz and within the Ecca Group. feldspar lithics) in the Fort Brown Formation.

Gridded aeromagnetic data was obtained from the South Africa (CGS). Airborne mag- 3.2 Density and porosity netic data was acquired over the Eastern Cape Province The average density of water was obtained to be about of South Africa in 1982. A proton precession magnetome- 1.002 g/cm3 at room temperature 25∘C. The calculated den- ter with 0.01 nT resolution was used to acquire the aero- sities of the rock samples are listed in Table 2. Bar charts magnetic data. The flight height and line spacing was60 showing densities and porosity of shales from the five for- m and 750 m respectively, while sampling along lines was mations are presented in Figures 5-9. Density values for all 250 m. The flight line direction was North-South with re- the Ecca Group rocks behave as expected (Figure 10) and spect to UTM Zone 35S and tie line direction of East-West. it can be depicted with the expression below: Temporal magnetic variation and IGRF were both removed from the magnetic data by CGS in order to have signatures ρdry < ρwet < ρgrain that are only due to the crustal magnetic field. The mag- netic data was reduced to the pole (RTP) and Geosoft Oasis The porosity result reveals that the Whitehill Formation Montaj was used calculate radially averaged power spec- is the most porous unit with an average porosity of about trum. Getech Getgrid software was used to compute the 2.1% followed by the Prince Albert Formation with a poros- magnetic depth slicing. The magnetic results are displayed ity of 1.26%, then the Fort Brown, Collingham and Ripon as geophysical maps. Formations, respectively (Figure 11). The high porosity ob- served in the Whitehill Formation can be explained by the fact that these rocks have undergone extensive weathering, thus minerals like pyrite has been altered to gypsum and 3 Results and Interpretation subsequently dissolve when in contact with water thereby opening pore spaces in rocks. The relationship between 3.1 Petrography the average densities and porosity of all the formations that make up the Ecca Group shows that they are inversely The rocks of the Ecca Group are both argillaceous and correlated. arenaceous rock. The argillaceous rock type are domi- nant in the lower Ecca Group (Prince Albert, Whitehill and Collingham Formations) while the arenaceous dom- 3.3 Magnetic inate the upper Ecca Group (Ripon and Fort Brown For- mations). The argillaceous rocks of the lower Ecca Group The magnetic data over the study area is shown in Fig- are mostly laminated with some mineral grains lying par- ure 12. The main magnetic anomaly in Figure 12 has ampli- allel to the cleavage (Figure 3). Thin and thick beds were tude of up to 120 nT and runs from west to east across the seen in areas without lamination. The framework miner- study area. This anomaly is a “bean shape” anomaly and als are quartz (monocrystalline quartz and polycrystalline could be the result of a buried body with high magnetic quartz), feldspar (albite, microcline and orthoclase), mi- minerals that stretches west - east across the study area. cas (muscovite and biotite) and lithics. The lithic grains This anomaly may be part of the Mbashe anomaly iden- are igneous, sedimentary, metamorphic and volcanic in tified and reported in [31]. The second long-wavelength Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 321

Figure 12: Reduced to the pole (RTP) magnetic anomaly map overlain on the mapped geology. anomaly is seen in the north-western part of the map. Sev- low arrows) are the main anomalies shown on the map and eral short-wavelength anomalies are evident in the north- are dominant in the northern part of the map. ern part of the study area in Figure 12, and are highlighted Depth to sources is connected to wavelength or fre- further in Figure 13 (analytical signal). quency of the potential field data. The depth slices high- The analytical signal map depicted in Figure 13 helps light specific interval of spatial frequencies (wavelength) to detect/locate structures like dolerite intrusions and that matches with the calculated or estimated depth in the faults because it is not affected by magnetic field inclina- subsurface by generating separate magnetic maps for a tion or declination, as well as the direction of source mag- sequence of horizons with increasing depth. It aids com- netization. Dolerite intrusions (e.g. sills, shown with yel- parison of amplitude response, thus the appearance and disappearance of a magnetic unit at different depth is 322 Ë C. Baiyegunhi et al.

Figure 13: Analytical signal map overlain on the mapped geology. recognised. Depth slicing works on the principle of Wiener pearance and disappearance of magnetic anomalous fea- filtering, which assumes that potential field signals (i.e. tures can be picked at a particular depth. Depth slice re- magnetic and gravity) results from two or more uncor- sults (slices 1 - 6) clearly shows that the main magnetic related arbitrary processes [32]. The various components anomaly in Figure 12 is due to a deeper source(s) because of the data can be separated by highlighting the effects the anomalous body appears in all the depth slices and of shallow sources from deeper sources [32]. Most mag- becomes broader with depth which is an indication that netic anomalies at the surface, usually originate at shallow the source is very deep and possibly located in the base- depth, while obscure magnetic anomalies usually origi- ment. Thus, it is suggested that the anomaly is due to a nate from a deeper depth [33]. Depth slices were performed buried magnetic-rich body within the basement. Also, a and shown in Figure 14 in order to compare the anomaly structural trend that coincides with the dolerite dykes and amplitude responses at different depths such that the ap- sills is seen in the far north-eastern part of slices 1- 5 (depth Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 323

Figure 14: Depth slice showing changes in anomaly features with depth. The top power spectrum plot is displayed with a logarithmic stretch in order to show the strength of the sine and cosine components at each frequency since lower frequency Fourier components usually dominate aeromagnetic datasets due to the enormous changes in the amplitudes of the different spectral components. The radially averaged power spectrum of the field increases with decrease in depth (h) by a factor that is proportional to exponential (−4πhk), where k is the wavenumber. 324 Ë C. Baiyegunhi et al. of about 1 – 10200 m) and thereafter disappear on slice 6 sibly affect or have affected the quality of the petroleum (depth of about 15800 m). The source of this trend is sug- resources. The magnetic map also shows a major mag- gested to be located between the depths of about 10200 – netic anomaly that trend in a west-easterly direction. This 15800 m. anomaly is a “bean shape” anomaly and could be the Mbashe anomaly that was documented by in [31]. Depth slice results (slices 1-6) clearly shows that this anomaly 4 Discussion is due to a deeper source(s) because the anomalous body appears in all the depth slices and becomes broader with depth which is an indication that the source is very deep The lower Ecca Group (the Prince Albert, Whitehill and the and possibly located in the basement. The depth that was Collingham Formations) were believed to have accumu- determined for this anomaly from the depth slices also lated in a deep marine basin, whilst the upper Ecca Group support the work of several authors that the “bean shape” (the Ripon and Fort Brown Formations) were deposited in a anomaly which coincides with a “shallow, sub-horizontal deltaic-lacustrine environment. It was confirmed through conductive band and regionally continuous in the Karoo mapping that the rocks of the Ecca Group are both argilla- basin” was located at a depth of about 7 – 30 km under- ceous and arenaceous rock. The argillaceous rocks are neath the surface [4, 35–38]. The dominant gravity varia- mainly shales interbedded with mudstone and occasion- tion in the study area is of long wavelength and it is sug- ally chert and claystones which are of future economic sig- gested to be a deeper source/interface inland that shallows nificance when considering the large pure clay minerals towards the e.g. the basement and/or Moho. in the study area. The sandstones are graywackes (mainly feldspathic graywackes with few quartzitic-wackes), im- mature and poorly sorted, therefore, giving an indication that the source area is near. The observed heavy miner- 5 Conclusions als as well as the lithic grains signify that the minerals are of granitic, volcanic and metamorphic origin. The Ecca The formations of the Ecca Group in the Eastern Cape Group density values range from 2.4 – 2.8 g/cm3. The Province of South Africa were investigated in order to re- porosity values range from 0.7 – 2.4%. The relationship veal their petrographic and geophysical characteristics. between the average dry density and porosity indicates The petrographic study reveals that the rocks of the Ecca that the two variables are inversely correlated. The poros- Group are both argillaceous and arenaceous rock. The ity results shows that the Whitehill Formation is the most argillaceous rock types are dominant in the lower Ecca porous unit with an average porosity of about 2.1% and Group while the arenaceous dominate the upper Ecca Ripon Formation has the lowest with an average porosity Group. The framework minerals are quartz, feldspar, mi- of about 0.8%. This density and porosity values fall within cas and lithics. The sandstones are graywackes, immature the range of 2.3 – 2.8 g/cm3 and 1 – 10% that was envis- and poorly sorted possibly indicating that the source area aged by several authors like [29, 30, 34] that studied the is near. The observed heavy minerals as well as the lithic formations that make up the Ecca Group. grains signify that the minerals are of granitic, volcanic Several short-wavelength anomalies (i.e. dolerite in- and metamorphic origin. Based on the proportion or ra- trusions) are evident in the northern part of the study area tio of the feldspar to lithic fragments, the sandstones of in Figure 13, and are highlighted further in Figure 14 (ana- upper Ecca Group are termed arkosic sandstones. The den- lytic signal map). These anomalies are also shown in the sity of the Ecca Group rocks varies from 2.525 – 2.782 g/cm3. depth slices result appearing in depth slice of about 6000 The average porosity range from 0. 75 – 4.4%. Based on the m below the ground surface. The targeted depth for uncon- depth slices result, the depth to the magnetic source of the ventional shale gas exploration in the Karoo Basin falls be- structural trend that coincides with the dolerite intrusions tween 3000 to 5000 m below the ground surface and hy- was suggested to be located between the surface down to drogeological investigation of Karoo aquifers are limited depths of between 10200 – 15800 m. These will have had to shallow depths of up to 500 m. As revealed by the depth a significant impact on the Whitehill Formation and there- slices, dolerite intrusions extend to a depth of about 6000 fore, the quantity of gas still in the basin. m and form a network of complex structures. The appear- ance of dolerite intrusions at targeted depth for gas explo- Conflict of Interests: The authors declare that there is no ration could pose a serious threat to fracking of the Karoo conflict of interests regarding the publication of this paper. for shale gas. Again, these dolerite intrusions could pos- Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown Ë 325

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